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Mar 20, 2013 - Tsung-Ying Tsai, Shoou-Jinn Chang, Senior Member, IEEE, Wen-Yin Weng, ... electrons could be emitted for the GaN NW field emitters with.
IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 4, APRIL 2013

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GaN Nanowire Field Emitters With the Adsorption of Au Nanoparticles Tsung-Ying Tsai, Shoou-Jinn Chang, Senior Member, IEEE, Wen-Yin Weng, Shuguang Li, Shin Liu, Cheng-Liang Hsu, Han-Ting Hsueh, and Ting-Jen Hsueh

Abstract— We report the adsorption of Au nanoparticles onto the surface of GaN nanowires (NWs) through photo-enhanced chemical reaction and the fabrication of GaN NW field emitters. With the adsorption of Au nanoparticles, it is found that threshold field and work function are reduced from 8.29 V/mm and 4.1 eV to 6.67 V/mm and 3.2 eV, respectively. These improvements could be attributed to the larger band distortion and more electrons accumulation so that more electrons could be emitted for the GaN NW field emitters with Au nanoparticles. Index Terms— Au nanowires.

nanoparticle,

field

emission,

GaN, Fig. 1.

I. I NTRODUCTION

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HE advantages of cold cathode field emitters over conventional adopted thermion emitters are their reduced electrical field and energy consumption [1]. Based on this property, various applications such as novel filters, sensors, and displays are developed [2], [3]. It is known that performances of field emitters depend strongly on the geometry of the material. With good aspect ratio, considerable attention is focused on the use of 1-D nanotubes or nanowires (NWs) as field emitters in recent years [4], [5]. With wide direct bandgap, GaN is now used extensively for blue/green light-emitting diodes (LEDs) [6], [7]. Other than thin-film GaN, GaN NW-based sensors, LEDs, and lasers are also demonstrated recently [8]–[10]. With low electron affinity and good physical/chemical stabilities, GaN NWs are also potentially useful for field emitter applications [11], [12]. It is shown that GaN NWs can be prepared by various methods such as vapor-liquid-solid (VLS), chemical vapor deposition, molecular beam epitaxy, thermal evaporation, and hydride-assisted growth [13]–[17]. Very recently, we reported the conversion of β-Ga2 O3 NWs to GaN NWs

Manuscript received December 23, 2012; revised February 6, 2013; accepted February 8, 2013. Date of current version March 20, 2013. The review of this letter was arranged by Editor T. Egawa. T.-Y. Tsai, S.-J. Chang, W.-Y. Weng, and S. Liu are with the Institute of Microelectronics and Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan (e-mail: [email protected]). S. Li is with the College of Science, China University of Petroleum (East China), Shandong 266580, China. C.-L. Hsu is with the Department of Electrical Engineering, National University of Tainan, Tainan 700, Taiwan. H.-T. Hsueh and T.-J. Hsueh are with the National Nano Devices Laboratories, Tainan 741, Taiwan. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2013.2247558

Schematic diagram of our field-emission measurements.

through ammonification and the fabrication of a GaN NW ultraviolet (UV) photodetector [8]. In this letter, we report the field emission properties of GaN NWs converted from β-Ga2 O3 NWs. Recently it is shown that Au nanoparticles could modify band structure and thus improved field emission of ZnO NWs [18]. Thus, we also adsorbed Au nanoparticles onto the GaN NWs. The differences between emitters with and without the Au nanoparticles will also be discussed. II. E XPERIMENTAL The GaN NWs are prepared by three steps: 1) in step 1, we grew β-Ga2 O3 NWs by heating a GaN/sapphire template through VLS method [19]; 2) in step 2, the β-Ga2 O3 NWs are converted to GaN NWs through ammonification [8]; and 3) step 3 is to adsorb Au nanoparticles. In this step, an ethanol solution of HAuCl4 ·4H2 O with ethanol: HAuCl4 ·4H2 O = 1000:1 is first prepared in a square alumina bath [20]. The GaN NW samples are then immersed in the solution and the alumina bath is subsequently placed in a Kinsten KVB-30D UV box. The solution and the alumina bath are then irradiated by UV light for 5 min to absorb the Au nanoparticles onto the surface of GaN NWs. During irradiation, the UV wavelength and power are kept at 380 nm and 100 W, respectively. Finally, the samples are annealed at 500 °C for 1 h in air to remove chlorine. For comparison, samples without the adsorption of Au nanoparticles are also prepared. III. R ESULTS AND DISCUSSION Fig. 2(a) shows top-view SEM image of the samples after step 1. It is found that high density, randomly oriented NWs are grown on the GaN/sapphire template. It is also found that average diameter of these NWs varied from 80 to 150 nm. In addition, it is found that an Au nanoparticle exists on the tip of each NW. Such an observation indicates that these NWs are grown by the VLS process. Fig. 2(b) shows XRD

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Fig. 2. (a) Top-view SEM image. (b) XRD spectrum of the sample after step 1. (c) Top-view SEM image. (d) XRD spectrum of the sample after step 2 as-converted NWs.

Fig. 3. (a) Top-view SEM image of the sample after step 3. (b) EDX spectrum measured from the surface of the stained GaN NWs. (c) Bright-field TEM image of the GaN NWs with Au nanoparticle. (d) High-resolution TEM image of individual GaN NW with Au nanoparticle in (c). SAED pattern in (d).

spectrum measured from the as-grown NWs. The sharp XRD peaks (104), (202), (111), (113), (313), (311), and (217) of β-Ga2 O3 (JCPDS Card No.11-0370). Such a result indicates that β-Ga2 O3 NWs are grown after step 1. Fig. 2(c) and (d) shows SEM image and XRD spectrum measured from the samples after step 2. It is found that the sharp XRD peaks observed in Fig. 2(d) can be indexed to (100), (002), (101), (102), (110), (103), (112), and (201) of GaN (JCPDS Card No.89-8624). In the presence of NH, NH2 , or H2 , it is possible to reduce Ga2 O3 into Ga2 O [21], [22]. The reduced Ga2 O can thus react with NH3 gas through the following equations: Ga2 O(s) + 2NH3 (g) ↔ 2GaN(s) + H2 O(g) + 2H2 (g); Gr = −33 − 0 · kJmol−1 (1) Ga 2 O(g) + 2NH3 (g) ↔ 2GaN(s) + H2 O(g) + 2H2 (g); Gr = −196 − 96 · kJmol−1

(2)

where G r is the Gibbs free energy. Such an observation indicates that the β-Ga2 O3 NWs as shown in Fig. 2(a) are converted to GaN NWs after ammonification. Fig. 3(a) shows SEM image of the samples after step 3. Compared with Fig. 2(c), it can be seen that nano-sized particles are adhered on the surface of the NWs. Fig. 3(b) shows energy-dispersive X-ray (EDX) spectrum measured from the surface of the stained GaN NWs. It can be seen that we observed a clear Au peak while no Cl-related signal is found. Such a result indicates that we have successfully adsorbed Au nanoparticles onto the surface of GaN NWs. Fig. 3(c) shows bright-field TEM image of the sample after step 3 while Fig. 3(d) shows HRTEM image of an individual GaN NWs with Au nanoparticle. These images clearly show single crystalline structure of the NW with an inter-planar spacing of 2.75 Å. This corresponds to the spacing of (101) planes of GaN [23]. The inset of Fig. 3(d) shows selected-area electron diffraction (SAED) pattern of the NW. The electron diffraction pattern also shows that the NW is crystalline. Fig. 4 shows field-emission characteristics measured from the fabricated emitters. During these measurements, the distance between anode and GaN NWs is kept at roughly 105 μm. It can be seen from Fig. 4 that the field-emission

Fig. 4. Emission current-voltage characteristics of the fabricated pure GaN NWs and GaN NWs with Au nanoparticle. The inset is F-N plots of ln(J /E 2 ) versus (1/E).

current increased slowly when the applied bias is low with a threshold voltage of 700 V (i.e., applied electric field E is 6.67 V/μm) for the GaN NW field emitter with Au nanoparticles. At this threshold, the emission current is only 8.88 × 10−4 mA/cm2 . As applied bias is increased to 8.29 V/μm, emission current increased exponentially to 2.81 × 10−2 mA/cm2 . It is also found that the fieldemission characteristics follow the Fowler–Nordheim (F-N) equation [12] ⎛ ⎞ 3/ −6 2 7 2 −6.83 × 10 φ ⎠ 1.54 × 10 F exp ⎝ (3) J= φ F where J is the current density, F is the local field intensity, and φ is the work function of GaN NWs. Knowing φ of pure GaN NWs is 4.1 eV, we can thus determine the fieldenhancement factor from β = F/E. To further investigate the field-emission behaviors, ln(J/E 2 ) is re-plotted as a function of 1/E, as shown in the inset of Fig. 4. The slope of the F-N plot shows that field-enhancement factor β of pure GaN NWs is around 341. It is known that β depends strongly on the distance between anode and the

TSAI et al.: GaN NANOWIRE FIELD EMITTERS

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Fig. 5. Schematic band diagram. (a) Pure GaN NWs. (b) GaN NWs with Au nanoparticles.

sample. With a rough surface, the distance between anode and GaN NWs should vary slightly. However, we still used 105 μm as the distance between anode and GaN NWs for simplicity. Since the surface morphologies of these two NWs are similar, we assume that β of GaN NWs with Au nanoparticle is also 341. With this assumption, it is found that work function φ of the GaN NWs with Au nanoparticle is 3.2 eV [24]. Fig. 5 schematically shows band diagram of the GaN NWs, where δ denotes the difference between Fermi energy and conduction band. While measuring the field-emission properties, a strong electric field is established between the NWs and the vacuum. The energy band of NWs is thus distorted due to the external field that resulted in narrowing of the tunneling barrier. As a result, the accumulated electrons could thus tunnel through the barrier. Knowing the work function of Au, it is found that δ of GaN NWs without and with Au nanoparticles adsorption are 1.4 and 0.5 eV, respectively, as shown in Fig. 5. Under strong external field, GaN NWs with Au nanoparticles that had smaller δ, should thus exhibit larger band distortion, higher carrier density, and the smaller voltage for accumulation. The variations imply that the Au coating increases the effective potential barrier height of GaN NWs [25]. As a result, electrons could be emitted much more easily. IV. C ONCLUSION In summary, we reported the adsorption of Au nanoparticles onto the surface of GaN NWs through photo-enhanced chemical reaction and the fabrication of GaN NW field emitters. It was found that threshold fields were 6.67 and 8.29 V/μm while work function were 3.2 and 4.1 eV for the GaN field emitters with and without the adsorption of Au nanoparticles, respectively. These improvements could be attributed to the larger band distortion and more electrons accumulation so that more electrons could be emitted for the GaN NW field emitters with Au nanoparticles. R EFERENCES [1] C. A. Spindt, “A thin-film field-emission cathode,” J. Appl. Phys., vol. 39, no. 7, pp. 3504–3505, Feb. 1968. [2] Y. Huang, Z. Wang, Q. Wang, C. Gu, C. Tang, Y. Bando, and D. Golberg, “Quasi-aligned Ga2 O3 nanowires grown on brass wire meshes and their electrical and field-emission properties,” J. Phys. Chem. C, vol. 113, no. 5, pp. 1980–1983, Jan. 2009. [3] N. Singh, C. Yan, and P. S. Lee, “Room temperature CO gas sensing using Zn-doped In2 O3 single nanowire field effect transistors,” Sens. Actuat. B, Chem., vol. 150, no. 1, pp. 19–24, Sep. 2010.

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